This invention relates to oxygen ion conducting electrolytes having enhanced oxygen ion transfer, and to electrochemical devices having an oxygen-rich electrode and an oxygen-deficient electrode in association with the inventive electrolyte, and more particularly to electrochemical devices including fuel cells having a multiplicity of internally exposed, oxygen ion transfer surfaces within an electrolyte zone to improve the oxygen ion transfer rate between the electrodes. More specifically, the invention relates to solid electrolyte fuel cells operable at reduced temperatures in the order of about 200.degree.-800.degree. C. and having a multiplicity of internal passages or pores to enhance oxygen ion transfer. The pores are sized to limit gas transfer but permit oxygen ion transfer along the surfaces of pores in a lateral direction from the oxygen electrode or cathode towards the fuel electrode or anode.
Typical solid electrolyte fuel cells utilize a porous fuel anode, a porous oxygen cathode and a crystalline electrolyte. Their operating temperatures are usually about 1000.degree. C. in order that an adequate quantity of oxygen ions are transferred per unit time across an oxygen ion transfer gradient in the electrolyte by "hopping" between oxygen vacancies within the crystal structure. Usually, the electrodes have good electronic conductivity while the electrolyte has poor electronic but good ionic conductivity. In general, these solid electrolyte fuel cells have been identified with problems in performance and natural stability associated with the high operating temperatures.
A number of developments have been carried out to reduce the high operating temperatures and other problems associated with these solid electrolyte fuel cells. In U.S. Pat. No. 4,024,036 a heteropoly acid represented by the formula EQU H.sub.m [X.sub.x Y.sub.y O.sub.z ]nH.sub.2 O)
has been disclosed as a proton permselective electrolyte. In the above formula, X includes a variety of metals including transition metals while Y is a transition metal but not the same as X.
In U.S. Pat. No. 3,300,344, a fuel cell is disclosed with a solid gas-impervious electrolyte compound of ZrO.sub.2 and Y.sub.2 O.sub.3 which has oxide vacancies for transfer of the oxide ion through the solid electrolyte, and with porous electrodes. For the cathode, a porous nonconductive matrix of the electrolyte composition is loaded with manganese oxide to improve electrical conductivity.
Other disclosures of interest may be found in U.S. Pat. No. 3,684,578; U.S. Pat No. 4,277,360, U.S. Pat No. 4,197,171 and U.S. Pat. No. 3,410,728.
At lower temperatures in the order of 600.degree.-800.degree. C., molten carbonate fuel cells with molten carbonates melting in the range of about 400.degree.-700.degree. C. are preferred over solid electrolyte fuel cells to achieve acceptable output current levels. The transfer of oxidant ions in the molten carbonate fuel cells is achieved by the initial formation of oxygen ions followed by their combination with carbon dioxide to form carbonate ions as a means of transferring the oxygen ions across the electrolyte. An initial conditioning of the cell is normally carried out at about 650.degree. C. for about one to two hundred hours (and usually about 50 hrs) to achieve normal performance levels associated with a plateau on the performance curve. After the conditioning period, the operation of the cell tends to be limited by the rate at which carbonate ions form. In general, the molten carbonate fuel cells have problems associated with the initial conditioning period, the maintenance of the molten carbonate, and corrosion of cell components by the molten carbonate.
One object of this invention is an oxygen transport electrolyte with enhanced oxygen ion transfer properties. A second object of this invention is an electrochemical device with a solid electrolyte structure providing oxygen ion transfer and operable at temperatures below about 1000.degree. C. A third object of the invention is a molten carbonate fuel cell operable at temperatures below about 800.degree. C. Another object of the invention is a molten carbonate fuel cell with an improved oxygen ion transfer rate. Yet another object of the invention is a fuel cell with a solid electrolyte. Still another object of the invention is a molten carbonate fuel cell operable at temperatures below about 800.degree. C. with reduced conditioning requirements to achieve plateau operating performance. An additional object of the invention is a solid electrolyte fuel cell operating in the absence of molten carbonate yet operable at temperatures below 1000.degree. C. and preferably below about 800.degree. C. A further object of the invention is a solid electrolyte fuel cell with desirable characteristics of oxygen ion transfer within a solid electrolyte structure. A further object of the invention is an electrochemical cell operating with a solid electrolyte and an oxygen-rich electrode and a second, oxygen-deficient electrode for gas sensing and electrolytic oxygen-pumping applications.